Interpretive Summary: Soybean seeds have high levels of nitrogen stored in protein. About 40% of a soybean seed is protein. This is used by seedlings in germination and early seedling growth. This is important for plant growth before the root system is large enough to acquire the nitrogen from the soil needed for the plant to grow. It is even more important if the plant is growing in nitrogen devicient infertile soils. This research shows that higher protein soybeans, those with more stored N in the seeds, grow better in poor soil nitrogen conditions than soybeans with lower levels on stored nitrogen.

Technical Abstract:
In low fertility conditions, germinating seedlings are heavily dependent upon N from seed reserves for growth and development. Experiments were conducted to examine the influence of seed N content and mobilization on leaf development and the ability to take up nitrate and up-regulate leaf growth processes during recovery from the period of N deprivation. Genetically homozygous soybean lines with a range of seed N contents were grown in hydroponics culture without an external N source. Microscopic analyses of the apical meristem and plant exposure to labeled 15N-nitrate allowed precise descriptions of changes in leaf initiation and nitrate uptake during recovery. In seedlings grown without N, inhibitions of leaf expansion and leaf initiation were detectible in the second week after germination, and both were inversely related to the amount of N released from the seed. The S/R ratio decreased, indicating early N stress, but the S/R adjustments were not proportional to the degree of N stress as occurs with older plants, suggesting limitations in C availability. When external N was supplied to plants after 15 or 23 days of N deprivation, the induction period for nitrate uptake was longer than that normally observed with N-replete seedlings, and rates of growth during recovery from the N stress were correlated with initial seed N contents. The inhibition of individual leaf expansion was released only if leaves were at an early developmental stage. Leaf initiation at the apical meristem immediately responded to the presence of external nitrate, with initiation rates approaching those for +N controls. The results indicate that seedling vigor in infertile conditions and the ability to respond to available N are strongly influenced by seed resources.